As shown in FIG. 1, a typical conventional input unit 1 for a programmable controller comprises input terminals INO and COM for connecting signal lines from a sensor 4, an LED indicator lamp 7 for indicating the presence of a signal, and an internal circuit 2 which is to be coupled to a programmable controller via signal lines not shown in the drawing, and receives input signals from the terminals INO and COM via a photo-coupler 2. The input unit 1 is further provided with terminals N and P for connecting a power source 6 for the internal circuit 3. The sensor 4 normally requires its own power source, but typically shares the power source 6 with the internal circuit 3 of the input unit 1. To accomplish the necessary wiring a terminal base 5 is used; this terminal base 5 is provided with two pairs of terminals N1 and P1, and N2 and P2, each pair being internally connected to each other, as shown in FIG. 1. The terminals N1 and P1 are connected to the different ends of the power source 6 while the terminals N2 and P2 are connected to the power input lines of the sensor 4 as well as to the common terminal COM of the input unit 1. In this way, the single power source can supply the necessary electric power to both the input unit 1 and the sensor 4, but a considerable external wiring work is necessary.
FIG. 2 shows a typical conventional output unit 8 which likewise comprises output terminals OUT and COM, a signal indicator lamp 14 consisting of an LED, and an internal circuit 9 which is coupled to the output terminals OUT and COM via photo-coupler 10. In this case, the load 11 consists of a lamp. The output unit 8 is further provided with a terminal +DC for receiving a DC power for the drive circuit 9a of the output unit 8. A single power source 13 supplies electric power to the internal circuit 9 as well as to the load 11 and the drive circuit 9a p of the output unit 8, but, again, a considerable external wiring work is required according to this conventional output unit 8.
According to a typical conventional arrangement of a programmable controller and interface devices, a programmable controller 15 and interface devices 16 are arranged in an upper part of the interior a control panel 14 adjacent to each other, and an elongated terminal base 18 having a plurality of screw-clamp type terminals is mounted on a lower part of the interior of the control panel 14 as shown in FIG. 3. And, they are connected together by multi-core cables 17.
In such a control system utilizing a programmable controller, the necessary wiring work is significant because a number of multi-core cables 17 must be passed inside the control panel 14, and a considerable wiring space is required.
Also in the case where the programmable controller 15 along with the interface devices 16 is centrally located, and the terminal bases 18 are distributed to various locations of a production line to which the programmable controller system is applied as shown in FIG. 4, the same situation exists, and a large number of multi-core cables must be passed between various parts of the controller system. Therefore, again, the wiring cost significantly increases the cost for the installation of the system, and a considerable space is required for the necessary wiring.